The interim president of Venezuela, Nicolás Maduro, may have a reputation as an approachable pragmatist but he has begun the post-Chávez era in the pugnacious style of his former boss – expelling US diplomats, accusing "historical enemies" of poisoning the president and labelling the domestic opposition as fascists who want to divide the country.

The question now is whether that rhetoric sets the tone of the next administration or is simply the opening salvo in an election campaign to decide the long-term successor to the leader who dominated the nation's politics for 14 years.

As the candidate for the ruling United Socialist party, Maduro, a former foreign minister, bus driver and trade union activist, will be the clear favourite but faces tough competition from an opposition that united last year around Henrique Capriles. But Maduro has the ultimate endorsement.

In his last public address before emergency cancer treatment, in December, Hugo Chávez spelled out clearly that the 50-year-old Maduro was his chosen successor.

"My firm opinion, as clear as the full moon – irrevocable, absolute, total – is …that you elect Nicolás Maduro as president," Chávez said in a dramatic televised statement. "I ask this of you from my heart. He is one of the young leaders with the greatest ability to continue, if I cannot."

With an upsurge of sympathy likely to be generated by the huge state funeral that is planned for Friday and endorsements from the many Latin American leaders who will come to pay their respects, Maduro has a good chance of securing a full six-year term. Although a long-term Chávez loyalist, Maduro was until recently known for having a very different persona; he was admired for his quiet demeanour and sensible pragmatism.

"He is a quiet and serious man. A very good orator and clear thinker," said Agustin D'Attellis, an Argentinian government adviser who has attended several meetings with Maduro. "But he is no Chávez."Perhaps hoping for a change, the US reached out to Maduro soon after he became vice-president – and effectively began running the country in Chávez's absence – and received a relatively concilliatory response.

However, Maduro's tone was very different on Tuesday. As well as expelling a US airforce attache accused of trying to foment a military plot, he accused imperial enemies – a term usually associated with the US – of poisoning Chávez. "We have no doubt that commander Chávez was attacked with this illness," Maduro said, echoing a charge first made by the former president himself. "The old enemies of our fatherland looked for a way to harm his health."

He said he would launch an investigation, but the US has denied the poisoning and plotting accusations.

Maduro's background suggests he will be a tough adversary. Born in 1952 and raised in Buenos Aires, he was a rock music enthusiast in his teens – with Led Zeppelin among his favourites – and reportedly considered a career in a band. Instead, he worked for the Caracas metropolitan transport system and became a union activist, who later went for training in Cuba. He retains close links with Cuba and is a friend of the Castros, which suggests he would not cut the subsidised oil supplies that Venezuela now provides to the island, as well as to several other left-leaning nations.

His second wife, Cilia Flores, a lawyer, is a former speaker of the national assembly. She led the legal team that helped free Chávez after the failed 1992 coup against the Pérez government.

The background of this couple suggests they are negotiators rather than confrontationalists, despite the bluff and bluster of Tuesday's press conference, which analysts said was aimed at a domestic audience.

Michael Shifter, head of the Inter-American Dialogue thinktank in Washington DC, said: "Maduro's ideas are not different, but his style is. He is union leader not a lieutenant colonel in the military.

"We can expect him to be rhetorically hardline, but he does have a pragmatic streak that we will see behind the scenes. He's someone that the US, and maybe even the opposition, can deal with.

"There will have to have two tracks: While he is hardline in public, he will have to make deals too to make up for the fact that he is not Chávez."

But nobody is expecting a change in ideology. After winning a seat in the national assembly, he helped revise the constitution and served in several key posts.

As foreign minister from 2006, Maduro was instrumental in forging ties with several of the US's pariah states – Libya, Syria and Iran. He was also credited with persuading Chávez that he should work with the Columbian president, Juan Manuel Santos, to end the long conflict with Farc armed revolutionaries.

Since becoming vice president opponents have mocked Maduro for doing a "bad imitation" of his boss. He has responded with the barbs of a class warrior, calling Capriles "the decadent prince of a parasitic bourgeoise".

As well as rampant inflation, murderously high crime rates and creaking infrastructure, the challenges for Maduro will including maintaining the unity of a ruling coalition, pulled together by the charisma of Chávez.

The absence of Diosdado Cabello, the speaker of the house who has close ties to the military, from Tuesday's televised press conferences, added fuel to rumours of a rift between the two men. But they have repeatedly dismissed such speculation and said they will work closely together to further the revolution begun by Chávez.